How would you set your opponent up to land your combination? I think that pre planned combinations makes for a predictable fighter. What happens when you're trying to throw your favorite combo and your opponent punches in between your sequences while avoiding your own punches? Understand that when you become focussed on just throwing a pre-planned combination that your attack can become a predictable pattern that a decent fighter can counter or avoid altogether. The same applies for any preset plan or preset anything.

Now look at guys such Julio Cesar Chavez, their punches may have looked like a simple combination that they just so happened to throw but actually it started out as a counterpunch. You see to them there's probably not a punch that their opponent throws that they can't counterpunch. A good counterpuncher will make their opponent throw the punch that they want to counter.

Combinations are great because it can spread out an opponent's defense especially against people assume a rigid defense such as the squared up hand high and tight defense that most fighters today are brought up in. Against an opponent with a nice flexible defense who decides to stay out of range and makes you do most of the leading, throwing a combination would be ineffective if you can't hit anything. Look at how Pernell Whittaker fights and think how would you be able to even lay glove on him. Boxing isn't always as easy as pick your spot and name your shot. The thing is that boxing at its highest level requires you to set up your punches in order to land anything. This can be practiced on the bags or in the ring, throwing counters off the slip, learning how to counter each kind of punch, and later on learning how to set your opponent up.